It is known that most polymer materials need to have a stabilization system in order to achieve satisfactory performance in their thermal, light, and oxidation stability. The additives are normally added to the polymer materials during melt processing, such as screw extrusion, injection molding, casting, blow molding, etc. The efficiency of these additives is not only a function of the additives themselves but also a function of the distribution of such additives in the polymer matrix. For example, U.S. Patent Application U2006/0155027 disclosed a composition in which reactive flame retardants are chemically bonded to the polymeric matrix so that the resulting polymer has a better flame retardancy and less blooming due to the fact that the functional groups are grafted on to the polymeric material matrix.
The additives can be chemically bound to the polymeric materials via grafting of functional additives by various methods known in the state of the art, such as by grafting reaction which may be conducted in polymer solutions, in the presence of solid polymer or with a polymer in molten state. The active sites on the polymers can be formed either in the presence of grafting additives, or be contacted with the functional additives at a later stage. The grafting sites can be produced by treatment with a peroxide or any other chemical compound which is a free radical polymerization initiator capable of extracting a hydrogen free radical from the polymer backbone, or by irradiation with high energy ionizing radiation.
The free radicals produced in the reaction as a result of the degradation of peroxides or by irradiation treatment act as initiators for polymerization of the functional additives, as well as active sites for grafting when the free radicals are formed on the polymer backbones. For example, U.S. Pat. No. 5,411,994 discloses a method for making polyolefin graft copolymers by irradiating olefin polymer particles and then treating with a vinyl monomer in liquid form under a non-oxidizing environment which is maintained throughout the process. U.S. Pat. No. 5,817,707 discloses a process for making a graft copolymer by irradiating a porous propylene polymer material in the absence of oxygen, adding a controlled amount of oxygen to produce an oxidized propylene polymer material and then heating, dispersing the oxidized polymer in water in the presence of a surfactant to react with a vinyl monomer by a redox initiator system.
In all the above-mentioned documents, the grafting reaction is carried out on the polymer in solid state, at a temperature lower than the softening point of the polymer itself. Graft copolymers have also been made in an extruder as disclosed in U.S. Pat. No. 3,862,265 in which an organic peroxide initiator was injected into the extruder to initiate the grafting reaction of polyolefins in molten state with vinyl monomers. The reactive extrusion, carried out on the polymer in molten state, offers many advantages such as a fast reaction rate and a simple reaction system. Nevertheless, such graft polymerization requires the use of organic peroxides during extrusion. Since peroxides are unstable and explosive chemicals, they require special safe handling procedures to minimize the risk. Moreover, the degradation products from the organic peroxide, such as t-butyl alcohol, undesirably remain in the final product and render the product unsuitable for certain applications. In addition, since the free radical initiator used in such a process does not only initiate the graft copolymerization but also homopolymerization of the vinyl monomers, relatively low grafting efficiency often occurs and results in low degree of graft monomer content, thus reducing the value of the final products.
There is a need, therefore, for a process for making a graft copolymer with a low by-product concentration and a stable starting material, easy to handle and not requiring unusual safety procedures. Accordingly, it is an object of this invention to produce a graft copolymer material without using an organic peroxide during grafting in order to eliminate the above-mentioned difficulties associated with the handling of organic peroxides and to avoid the toxic by-products resulting from their use.